Electrostatic precipitator



Sept. 10, 1968 R. H. BOLL 3,400,513

- ELECTROSTAT IC PRECIPITATOR 2 Sheets-Sheet 1 Filed Sept. 8. 1966 FIG.1

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[ ENTE LIN E .R T\ K WXEL RADIAL FIELD AT WALL INVENTOR. Richard H. Boll6 AT TORNEY R. H. BOLL ELECTROSTATIC PRECIPITATOR Sept. 10, 19 68 2Sheets-Sheet 2 Filed Sept.

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H n n United States Patent 3,400,513 ELECTROSTATIC PRECIPITATOR RichardH. Boll, Alliance, Ohio, assignor to The Babcock & Wilcox Company, NewYork, N.Y., a corporation of New Jersey Filed Sept. 8, 1966, Ser. No.578,435 6 Claims. (Cl. 55-103) ABSTRACT OF THE DISCLOSURE Anelestrostatic precipitator having an open ended housing of dielectricmaterial with hoods positioned on said opposite ends of the housing todirect a flow of dust laden gas into and clean gas from theprecipitator. The dust in the gas is electrostatically charged uponentering the housing and the dust collected after leaving the housingwhile the electrostatic charge on the dust is increased byelectrogasdynamics in passing through the housing for enhanced dustcollection.

The present invention relates to an improved electrostatic precipitatorand more particularly to a precipitator the performance of which isimproved by utilization of the electrical potential generated by amoving gas stream in apparatus adapted for applying electrogasdynamicprinciples so that dust collection in the collector section of theprecipitator is enhanced and the electrical power requirements areminimized.

The use of electrostatic principles for separating dust from gases iswell known. In this instance the dirty gas contains dust particles whichare cleaned by the action of the electrostatic force which acts oncharged particles within an electrostatic field. The particles areusually charged by passing them through or near the region of coronadischarge, and precipitation occurs when the gas stream passes through aregion containing an electrostatic field. The latter region may beeither identical with or separated from the region of particle charging,but in any event the electrostatic field is supplied by an externalpower supply. This precipitating power supply is usually of highervoltage than actually required for particle charging and is the sourceof several practical problems. For example, because of its high voltage,the precipitating power supply is expensive; arcing between precipitatorelectrodes presents the possibility of current surges whose effects mustbe guarded against by elaborate electronic control. Moreover, if asingle electrode pair shorts out completely due to dust build up andarcing, a large section of the precipitator is rendered inoperablebecause all of its elements are fed from a common power supply which isshorted. While these difficulties can be overcome to a degree inconventional practice by various means, they nevertheless result inincreased cost, lower reliability, and loss of precipitating efiiciency.

In the present invention, I utilize a corona discharge for charging theparticles in much the same fashion as accomplished in conventionaltwo-stage precipitators. This charging process, which occurs in anionizer section, requires a power supply of modest voltage (e.g., 5,000to 10,000 volts) and, therefore, modest cost compared to that requiredby a single-stage precipitator. However, I achieve precipitating fieldsof magnitude comparable to those obtained in a single-stage precipitatorwithout employing a corresponding external power supply. The voltagerequired for the precipitating field is generated internally by aplurality of electrostatic generators operating on electrogasdynamicprinciples and using the gas containing charged particles as the workingfluid. Thus, not only is the cost of high voltage power supply avoided,but operating difficulties due to current surges 3,400,513 PatentedSept. 10, 1968 and short circuits are eliminated. For example,interelectrode arcing causes no damage to the apparatus and only verybrief and slight impairment of collection efficiency; even completeshort-circuiting of a few electrode pairs causes only slight decrease inoverall collection efiiciency because of the large plurality ofelectrogasdynamic generators and collectors that are placed in flowparallel. Additionally, such high voltages are attainable byelectrogasdynamic principles that relatively large interelectrodespacings can be utilized, which results in an additional cost advantage.Moreover, since maximum particle charging is not essential, overallpressure drop can be held to not greater than about one inch of water.

Electrogasdynamic generation of the high precipitating voltage isachieved using the dust particles themselves as charge carriers. Thedust-laden gas is first passed through an ionizing section wherein thegas velocity is increased somewhat and particle charging occurs bycorona discharge between a number of corona needles and oppositelydisposed attractor electrodes'Thereafter, the gas flows into adielectric conduit wherein the electrostatic potential increasesparabolically in the flow direction due to the space charge whichresides on the charged dust particles. At a suitable distance down thischannel, the high electrostatic potential is communicated to a series ofneedle electrodes which are electrically connected to repulserelectrodes located still further downstream. Between the region of theneedle electrodes, which I call repulser corona needles because a coronawill sometimes form between them and the highly charged dustladen gas,there is a continuation of the dielectric flow conduit and theelectrostatic potential falls again towards zero. Actual particlecollection takes place within a collector section at the end of thedielectric section, wherein are placed a multiplicity of groundedcollector electrodes. The electrostatic field necessary for particlecollection is generated by the potential difference between repulserelectrodes, which are placed within or adjacent to the collectorelectrodes, the repulser electrodes being electrically connected to therepulser corona needles. Thus, by virtue of the space charge on the dustparticles, there exists between the ionizer and collector sections aparabolic electrostatic potential, which may achieve a quite high peakvoltage; the repulser corona needles are situated approximately at theplane of maximum electrostatic potential, and they achieve potentialsonly slightly lower than the maximum electrostatic potential by means ofcorona discharge from the particles to the needles. This charge transferfrom particles to the repulser electrodes occurs mainly during startup;once the repulser electrodes have attained full potential, there is nocurrent drain from them, except for that due to an occasional spark.Such electrical power as is dissipated in occasional spark dischargingof the repulser electrodes is generated from the gases by their forcingthe charged particles against the electrostatic field in the upstreampart of the generator section; if no discharging occurs, such fluidpower as is required to push the charged particles against theelectrostatic field in the upstream portion of the generator is returnedto the fluid from the charged particles by their pushing the gas streamin the downstream portion of the generator; but in any event, thepressure drop in the generator section is less than a few tenths of aninch of water because of the large volumetric flow of gas and thecomparatively small amount of electric power required.

Preferably, the collector section is provided with a water washingarrangement for removing the collected dust from the surfaces of thecollector electrodes. Alternately, mechanical means may be used toremove collected dust; however, the type of dust removal means will belargely dependent upon the temperature of the gases a cleaned by theelectrostatic precipitator of the present invention.

The various features of novelty which characterize the invention arepointed out with particularity in the claims annexed to and forming apart of this specification. For a better understanding of the invention,its operating advantages and specific objects attained by its use,reference should be had to the accompanying drawings and descriptivematter in which there is illustrated and described a preferredembodiment of the invention.

Of the drawings:

FIG. 1 is a schematic showing of the electrostatic precipitator of thepresent invention;

FIG. 2 is a diagramatic showing of the electrical characteristics of thevarious sections of the precipitator; in relation to the gas beingtreated;

FIG. 3 is a schematic illustration of a precipitator constructedaccording to this invention;

FIG. 4 is a section taken along the line 44 of FIG. 3; and

FIG. 5 is an enlarged section of a single element of the unit shown inFIG. 3.

As illustrated in FIGS. 1 and 2 a single element of an electrostaticprecipitator constructed according to the present invention includes adielectric body or housing enclosing the precipitator element where thehousing may be constructed of plastic, fiberglass, or other ceramicmaterials having the desired dielectric characteristic. Isopotentiallines Z are shown qualitatively to help envision the electrostaticfields involved. As shown, the incoming gases enter the housing as at Y,and are accelerated in passing through a restricted throat 11. Thethroat is formed with a metallic insert 12 defining at least part of thewalls of the throat, and is provided with a centrally located coronaneedle 13 which projects substantially through the full extent of thethroat 11. As shown, power from a DC source 14 is applied to the inserts12 through a connection 15 and through the connection 16 to the coronaneedle 13. The supply to the two components being of oppositepolarities.

The dirty gases in passing through the ionizing section 17 are partiallyionized and the dust therein contained is electrostatically charged witha polarity corresponding to that of the ionizer needles. Thereafter thegases proceed through a passageway 18 of increasing cross-sectional flowarea which terminates in a collector section 20. The walls of section 20are defined by collector electrodes 21 connected through line 22 toground potential. Intermediate the ionizing section 17 and the collectorsection 20 and within the passageway 18 is positioned a repulser coronaneedle 23, which is exposed to the ionized gases passing thereover andis connected with a repulser electrode 24. Electrode 24 isconcentrically arranged or centrally located with respect to andco-extensive with the collector electrode 21 and connected with therepulser corona needle 23 by an electrically conducting wire 25.

The electrostatic potentials and fields of the apparatus of FIGURE 1will be qualitatively as indicated in FIG- URE 2 during operation. Itwill be noted that the center line electrical potential resulting fromthe electrogasdynamic efiect due to the gases flowing through thepassageway 18 will provide a substantial voltage on the repulserelectrode 24 resulting in a very substantial increase in the radialelectrostatic field between the repulser electrode 24 and collectorelectrode 21, thus providing a high dust-collecting effectiveness.

As shown in FIGS. 3-5 a multiple arrangement of precipitator units maybe used to process gas quantities exceeding the capacity of one unit. Insuch a commercial installation, the housing surrounding the precipitatoris formed of dielectric material and is provided with a hood 31 of steeladjacent the upper end of the housing and with a steel outlet hood 32adjacent the outlet end of the housing. The connections from externalpower source 26 include a connection 27 to each of the ionizer elementsor bars 33 situated in the gas restrictor or throats 34 positioned inthe upper end of the housing 30. The opposite side of the power sourceis connected to ground, and to the inlet and outlet hoods by line 28.

As shown particularly in FIGS. 3 and 5 the restrictors or throats 34adjacent the inlet end of the housing 30 are formed in the general shapeof Venturi tubes, with the metal ionizer bars 33 imbedded in thedielectric material which is shaped to define the throats 34. Coronaneedles 36 are suspended from above by Wires connected to support grid37 and are arranged to extend into the throat of the associatedrestrictors so as to be concentrically positioned with respect to thebars 33. Corona needles 36 are electrically grounded by connectionthrough the wires 35, support grid 37, and inlet duct 31.

- The depending ends of some or all of the corona needles are providedwith bars or strands 38 of a dielectric material, such as aluminumoxide, to provide support for subjacent repulser corona needles 40positioned intermediate the ionizing section A and the collector sectionB of the unit. Each of the repulser corona needles 40 is connected by anelectrically conducting wire 41 to a repulser electrode 42 suspended inthe collector section B. The repulser electrodes 42 are concentricallypositioned relative to the collector electrodes 43 in collector sectionB whether the cross-sectional shape of the electrodes is circular,square, or of other cross-sectional configuration. Although not shown inthe drawings, the corona needle 40 and repulser electrodes 42 may ifnecessary be supported by a grid or grids comparable with the supportgrid 37 supporting the corona ionizer needles 36 since it is desirableto maintain a generally fixed concentric relationship between eachrepulser electrode 42 and its corresponding collector electrode 43 so asto avoid electrical shorting of the assembly.

An enlarged view of an individual precipitator unit of FIG. 3 includingthe needle and repulser electrode assembly is shown in FIG. 5. It willbe noted that both FIGS. 3 and 5 illustrate an arrangement whereby waterintroduced through pipe 39 as in FIG. 3 is utilized to wash thecollected dust from the collector electrodes 43. This is accomplished byutilizing riser passageways 44 so that the water will flow, generally infilm form, over the upper end of the collector electrodes 43 and thendownwardly over the surface of each electrode. As shown particularly inFIG. 3 the water, with the collected solids, discharges downwardly fromthe collector section B into a sump 45 formed in the outlet hood 32. Themixture of water and collected solids is removed from the sump through apipe 46 as required for discharge to Waste, or if the solids are of atype having industrial values, the sump discharge may be transmitted tosettling tanks and the like for separation and reclamation of the solidsfrom the liquid.

In operation the dust laden or dirty gases entering the upper hood 31 ofthe electrostatic precipitator as indicated at X in FIG. 3 passsuccessively downward over the support grid 37 and through the throats34. In passing through the throats of ionizer section A the gases aretemporarily ionized and the dust particles electrostatically charged. Inthe course of their continuing flow through passageway C, the gasescontinue to build up electric potential in accordance with knownelectrogasdynamics principles prior to entering collector section B. Inthe collector section B the collector electrodes are spaced from eachother laterally to provide passages for flow of water therethrough, thedischarge being generally as a film flowing over the surfaces todislodge the accumulated dust particles from the collector surfaces.Thereafter the water and separated dust is discharged to the bottom ofthe precipitator for subsequent disposal. As previously pointed out, theshell or housing 30 of the generator section is formed of dielectricmaterial which may be a ceramic, formed of fiberglass or various formsof plastic, for example, so that the static electricity built up withinthe generator portion will have a mainly axial electrical field.Ordinarily no appreciable surface discharge from the dielectric housing30 will occur and thus the axial field will develop on account of thesurface charge represented by the charged dust particles and such ionsas may be present and in transit through'the precipitator element. Thepotential distribution will in general follow the form illustrated inFIG. 2. The arrangement is such that there will be a minimum probabilityof an electrical breakdown or leakage between the repulser electrode 42and the ground. This is due to the spacing of the electrodes 42 and therepulser needles 40 with respect to wall leakage of the housing 30 andto the manner of support of the electrodes 42, i.e. by suspension fromcorona needles 36 and support grid 37. It is realized that leakagebetween the repulser needles 40 and/ or the repulser electrodes 42 andthe housing 30 may occur at infrequent intervals, with consequentdischarge of high potential electricity by means of short circuitthrough the gas within the system. Nevertheless the potential chargewill immediately build up to an effective separating charge so that suchshort circuiting will not appreciably effect the efliciency of dustcollection.

It will be noted in the FIG. 3 showing of the apparatus that there willbe more than one corona needle 36 in the ionizer section for eachrepulser electrode in the collector section. This is due to thedesirability of maintaining adequate spacing in the collector sectionbetween the repulser electrode 42 and the collector electrodes 43. Thusin installation there may be from 24 or even more, corona needleelectrodes 36 in the ionizing section for every repulser electrode 40 inthe collector section.

It should be realized that the power charge initially applied to theionizer section A will be of the order of 10,000 volts direct currentwith relatively low amperage. By using the electrogasdynamic generatorprinciple within the generating section C the static electricity willordi narily build up to a range of from 50,000 to 200,000 volts in thecollector section B with consequent enhanced collection efliciency inthe apparatus. Thus the present electrostatic precipitator utilizesconsiderably less external power for comparable collection efficienciesthan has been usual in the ordinary electrostatic precipitator.Likewise,

' arcing across the electrodes will not seriously affect or interruptthe power supply to the precipitator. For any arcing that may occur, theduration is short and infrequent, and does not adversely affect thepower to the ionizer section A. Furthermore, efficient electrostaticseparation can be attained without excessive gas flow velocity and highpressure drop in the apparatus. In apparatus such as disclosed thepressure drop through the system will be of the order of but one inch ofwater, which is comparable with that of conventional electrostaticprecipitators having comparable capacities. On the other hand, theamount of material utilized in the electrostatic precipitator in thepresent invention is considerably less than that previously considerednecessary in electrostatic precipitators of comparable capacity so thatthe present invention is more economical both to install and to operatethan the precipitators which have been utilized heretofore.

While the precipitator described is limited in the temperature of thegases it can handle, due to the use of water washing in the collectorsection B it will be understood that higher gas temperatures can beaccommodated using other means for removing the collected dust from thecollector electrodes. For example, such dust removal could beaccomplished by wrapping or scraping of the electrode surfaces 43.However, for most normal installations the described apparatus issatisfactory and will efficiently remove the dust from the gas with verymodest expenditure for external power supply, minimum cost of electricalpower consumed, negligible difficulty due to interelectrode arcing, andcontinuously maintained high efficiency in spite of severalinterelectrode short-circuits.

While in accordance with the provisions of the statutes I haveillustrated and described herein the best form and mode of operation ofthe invention now known to me, those skilled in the art will understandthat changes may be made in the form of the apparatus disclosed withoutdeparting from the spirit of the invention covered by my claims, andthat certain features of my invention may sometimes be used to advantagewithout a corresponding use of other features.

What is claimed is:

1. An electrostatic precipitator comprising an open ended elongatedhousing formed of dielectric material, an inlet hood formed ofconductive material and attached to one end of said housing for passingdust-laden gas into said housing, an outlet hood formed of electricallyconductive material attached to the opposite end of said housing forpassing dust-free gas out of said housing, charging means positioned inthe inlet end portion of said housing for electrostatically charging theparticles of dust in the dust-laden gas, a plurality of transverselyspaced repulser and collector surfaces positioned in said outlet hoodand adjacent said housing for electrostatically collecting the dust fromsaid gas, and a plurality of transversely spaced repulser corona needleswithin said housing and spaced apart a substantial distance from saidcharging means, said repulser corona needles being intermediate thelength of said housing and between said charging means and said outlethood for increasing the electric potential between said repulser andcollector surfaces by electrogasdynamics to enhance the collectingeffect of said repulser and collector surfaces, a plurality ofelectrically conductive wires, each of said wires connecting arespective one of said corona needles with a respective one of saidrepulser surfaces, means connecting said inlet hood, said outlet hoodand said collector surfaces to an electrical ground, means connectingsaid charging means to a source of high voltage direct currentelectrical potential, and support means within said housing andoperatively associated with each of said repulser corona needles, saidwires and said repulser surfaces for supporting said repulser coronaneedles, said wires, and said repulser surfaces out of contact with saidcollector surfaces.

2. An electrostatic precipitator according to claim 1, wherein saidelectrostatic charging means includes a gas flow restrictor foraccelerating the gases entering said housing, and a corona needlecoaxially extending through said restrictor.

3. An electrostatic precipitator according to claim 2, wherein saidhousing is vertically oriented and gas flow is downwardly therethrough.

4. An electrostatic precipitator according to claim 3, wherein saidrepulser needle is suspended from an upwardly adjacent corona needle bysaid support means which comprises a dielectric strand.

5. An electrostatic precipitator according to claim 3, wherein washingmeans are provided to Wash the dust from said collector surfaces.

6. An electrostatic precipitator according to claim 1, wherein means areprovided for removing the collected dust from said collector surfaces.

References Cited UNITED STATES PATENTS 1,337,489 4/ 1920 Strong.2,142,128 1/1939 Hoss et a1. 2,615,530 10/1952 Hodson et a1. 2,662,60812/1953 Fields 55-137 2,715,944 8/1955 Dohrer 55--113 X 2,789,657 4/1957Fields 55-437 2,798,572 7/1957 Fields 55137 2,813,595 11/1957 Fields55-137 2,814,360 11/1957 Beaver.

(Other references on following page) UNITED STATES PATENTS FOREIGNPATENTS McDonald et a1. 55137 792,068 3/1958 Great Britain.

Roos er 55 137 820,415 9/1959 Great Britain. Vicard.

De Seversky- 5 HARRY B. THORNTON, Primary Examiner. 32:32: at D.TALBERT, Assistant Examiner.

Weindel et a1.

